FIG. 1 is illustrative of an exemplary known generator 20, which includes a rotatable rotor 22. The respective axial ends of the rotor are circumscribed by rotor retaining rings 24. The generator 20 includes a stator core 26, which defines a cylindrical bore 28, circumscribing the rotor 22. The core bore 28 has a generally constant diameter portion 30, except at its axial ends, where the diameter increases along the axis towards each respective end. The increase in bore diameter is stepped, so the portion adjacent to each axial end of the stator core is referred to as “step iron” 32. The step iron portion 32 facilitates in reducing the magnetic flux densities at the ends of the stator core 26. The rotor 22 and stator core 26 are radially separated by a rotor gap 34, which is accessible from an axial end of the generator 20 in the annular volume between the step iron 32 and the rotor retaining ring 24.
As described in U.S. Pat. No. 8,823,412, which is fully incorporated by reference herein, the stator cores 26 of electrical generators 20 and other electrical machines are made up of a stack of several, for example, thousands of individual steel sheets or laminations 36. The thickness of an individual sheet 36 is typically measured in thousandths of an inch. Each of the laminations 36 is coated with a thin layer of electrical insulation to insulate it electrically from its neighbors. This insulation prevents the alternating magnetic flux in the stator core 26 from inducing eddy currents between laminations 36. If the insulation between adjacent laminations 36 becomes damaged during assembly, operation, or maintenance, a conducting path may be formed through which currents are induced by the alternating flux. These conducting currents create “hot spots” which, if undetected, can result in damage to the machine.
One type of apparatus suitable for detecting such stator core hot spots is an Electro-magnetic Core Imperfection Detector (“EL CID”). An EL CID system detects potential hot spots electromagnetically by exciting the stator core 26 with an externally supplied current source and then measuring any resulting fault currents flowing through a damaged region. Typically, the EL CID system's sensors are held against the stator core laminations 36 inside the stator core bore 28, where the laminations 36 define the stator bore 28 circumference. The system sensors typically are then translated or transported along the axial length of the stator core 26, though other translation paths may be chosen. As the sensors scan axially along the length of the core, they produce an analog signal having a magnitude proportional to detected fault currents. By examining a plot of the magnitude of the analog signal versus the sensor distance along the length of the core, operators are able to detect the location of a damaged insulation.
During inspection using an EL CID system, the scanning operation at the main section of the stator core 26, (i.e., the cylindrical portion having constant diameter 30), may carried out by an automated tool, for example by using a belt driven carriage for the sensor riding on the inside diameter of the stator core 26 in an axial direction, as shown in U.S. Pat. No. 4,803,563. However, in the step iron portion 32 of the stator core 26 it is difficult to obtain accurate readings from the sensor due to the abrupt changes in contour axially over that region. The stepped changes in the region's contour make it difficult to move the sensor over that region by use of a carriage or manually, to avoid distorted outputs. A source of output distortion is the inability to maintain constant relative distance and alignment of the stator core bore 28 circumferential surface and the scanning sensor. It is more difficult to avoid distorted EL CID scanning sensor outputs when an automated tool of the above-mentioned type of U.S. Pat. No. 4,803,563 is used to move the sensor over the step iron portion 32, though the tool advantageously facilitates inspection of the constant diameter axial portions 30 of the stator core 26 while the generator rotor 22 remains in situ.
The EL CID system described in U.S. Pat. No. 8,823,412 inspects the stator core step iron region and any other part of the stator core bore. However, unlike the EL CID system shown in U.S. Pat. No. 4,803,563, it does not perform a stator lamina insulation inspection with an in-situ rotor.